The invention pertains generally to a quadrature partial response communication system and is more particularly directed to a quadrature partial response demodulator having increased demodulating efficiencies.
Quadrature partial response communication systems are known in the art for transmitting digital data over transmission links. The QPR communication systems usually comprise a QPR modulator which modulates digital information into a form which can be transmitted over the transmission link to a QPR demodulator where the digital information is recovered.
The modulation process which is termed quadrature partial response (QPR) is a combination of techniques. Partial response refers to the process of digital communications where a predetermined amount of one symbol affects the next symbol. One particular partial response communications process for transmitting binary numbers is duobinary where three levels (positive and negative levels about a zero reference) are formed. The scheme is robust and provides a high bit rate transfer with a low bandwidth utilization. When phase modulated on a carrier a duobinary signal can be combined in quadrature (90.degree. out of phase) with another similarly modulated duobinary signal. This quadrature modulation retains the efficient bandwidth of the duobinary signal while doubling the bit rate.
One of the more elegant schemes of duobinary is to have: EQU y.sub.k =x.sub.k +x.sub.k-1
where y.sub.k is the encoded value of a symbol at time k and is found by taking the present symbol value x.sub.k at time k and adding to it the symbol value x.sub.k-1, one symbol period earlier. If the input data stream is binary having elements {-1, 1}, then the output data stream is tertiary having elements {2, 0, -2}. Conversely, if y.sub.k is the encoded symbol transmitted by the communication system, then: EQU x.sub.k =y.sub.k -x.sub.k-1
where x.sub.k is the decoded symbol value and the difference y.sub.k -x.sub.k-1 is the present encoded value minus the previous decoded value.
From the above decoding algorithm for duobinary it is evident that the demodulator is correlative, i.e., the process of decoding a transmitted bit value y.sub.k requires the knowledge of the previous bit value x.sub.k-1. While this provides many benefits, the correlative may cause errors to propagate from one bit to the next. The probability of an error propagating from one bit to the next bit is 1/2 and the probability of an error propagating through a number of multiplexed channels is related to the spacing between bits of an individual channel. By multiplexing even a small number of channels together, the probability of an error propagating in any one channel can be made very small, for example, in six channels the probability becomes 1/2.sup.6. However, when QPR modulation takes place there is a demultiplexing of channels into the I and Q phases which for an even number of channels removes much of the error protection of the original multiplexing. In a normal six channel QPR communication system half the channels are demultiplexed into one phase and the other half into the other phase of the QPR signal. This increases the error propagation probability dramatically to 1/2.sup.3. It would, therefore, be extremely advantageous to provide a multiplexed QPR communication system which provided the advantages of QPR modulation while it retained the benefits of maximum channel bit separation to reduce error propagation probability.
A preferred technique for demodulating QPR modulated information is the decision feedback circuit. In the decision feedback demodulating scheme, an amplitude modulated partial response signal is input to a mixer and multiplied by a carrier which is coherent with the modulated data carrier. The signal is then split into two channels I and Q in which decision feedback circuits can be used to demodulate the digital data contained therein. Many of these decision feedback demodulators are extremely complex and do not provide precision decoding without undue expense. What is needed is a decision feedback demodulator for a quadrature partial response signal which is simple and inexpensive, but which accurately performs the demodulation and decoding of the information on the QPR signal.
The coherent demodulation carrier of a quadrature partial response signal can be recovered or extracted by a number of different circuits but one of the more elegant methods for the generation of such carrier is the Costas cross over loop. The implementation of the Costas loop is problematic because it is normally necessary to use expensive multipliers which additionally require precision gain control through the loop. This requirement makes the circuit much more complex than it should be to phase lock the demodulation carrier to the carrier of the QPR signal. What is needed is a carrier extraction circuit for a decision feedback demodulator of a QPR signal which is simple and inexpensive but is accurately performs the extraction of a demodulation carrier which is coherent with the QPR carrier.